Pharmacokinetics and metabolism of fenbendazole in channel catfish

Fenbendazole (FBZ) was administered intravenously (1 mg/kg) and orally (5 mg/kg) to catheterized, confined channel catfish. Blood samples were collected for 72 h, and resulting FBZ plasma concentrations were pharmacokinetically modelled. Following intravenous administration t 1/2 alpha was 0.51 h, t 1/2 beta was 16.8 h, body clearance (C1B) was 0.0598 L/kg/h, and Vd (area) was 1.45 L/kg. After oral administration the t 1/2 (abs) was 1.47 h, the t 1/2 beta was 20.1 h, and the tlag was 0.1 h. Following oral administration of 5 mg FBZ/kg body weight, the following tissues and body fluids were sampled for concentrations of FBZ, oxfendazole (FBZ-SO), sulphone metabolite (FBZ-SO2) and hydroxy metabolite (FBZ-OH): liver, posterior kidney, fat, muscle, bowel contents and urine. Fenbendazole was detected in the highest concentrations in abdominal fat, whereas oxfendazole was found primarily in the kidney, liver and abdominal fat. The sulphone metabolite was detected only in urine and bowel contents, while the hydroxy metabolite was found most often in the liver and abdominal fat samples.